Contrasting effects of different types of resource depletion on life-history traits in Daphnia

1. Life-history traits of animals are known to change according to food quantity, but there is another type of resource depression, namely depletion of food quality. To examine life-history responses of herbivores to changes in resource quality, Daphnia obtusa Kurz, a common planktonic herbivore, were reared individually under several algal food regimes with different quantity and quality in terms of chemical composition. 2. Compared with Daphnia fed on a small quantity of nitrogen (N) and phosphorus (P) sufficient food, Daphnia fed on a large amount of P deficient food grew more slowly at young ages but continually grew until older ages. This trend was also found for Daphnia fed on a large amount of N deficient food. 3. However, the size-specific reproductive investment (measured as biomass accrued per unit time) of Daphnia fed on a large amount of N or P deficient algae was similar to those fed on a small amount of N and P sufficient food, indicating complex life-history responses to food quality and suggesting that nutritional requirements for reproduction differ from somatic growth. 4. Compared with individuals fed high-quality and high-quantity food, Daphnia fed on a large amount of P deficient food produced smaller eggs, while individuals fed on a small amount of P and N sufficient food produced larger eggs. Furthermore, 15-30% of eggs produced by individuals fed on P or N deficient food ceased to develop and were aborted. 5. These results demonstrate that life-history responses to food quality differ from responses to food quantity. Since the chemical composition of algae is highly variable in nature, herbivorous plankton demographics may not be predictable from resource quantity alone.